This invention relates to a surface acoustic wave device operable in a high-frequency band and, in particular, to a surface acoustic wave device comprising a ZnO piezoelectric thin film layer formed on a diamond layer or a substrate with a diamond film formed thereon.
A conventional surface acoustic wave device comprises a diamond layer or a substrate with a diamond layer formed thereon, an Al (aluminum) electrode formed on the diamond layer, and a ZnO (zinc oxide) piezoelectric thin film layer formed on the diamond layer and the Al electrode. The conventional surface acoustic wave device has a propagation velocity as high as 10,000 m/s and is therefore useful for exciting a high-frequency surface acoustic wave of, for example, a center frequency of 2.5 Ghz or the like when an array of electrodes Is arranged at an electrode period corresponding to a wavelength (for example, 4 .mu.m) of the surface acoustic wave. Practically, the electrode period may be substantially equivalent to the wavelength of the surface acoustic wave.
In order to achieve a similar high-frequency operation, another conventional surface acoustic wave device comprises a single crystal piezoelectric substrate and a metal thin film electrode formed on the substrate. With this structure, a very fine machining process is required to form electrode fingers of a dimension on the order of submicrons. For example, the electrode period must be as small as 1.3 .mu.m and 1.6 .mu.m in an ST-cut quartz substrate and a 128.degree. Y-X LiNbO.sub.3 (lithium niobate) substrate, respectively.
On the other hand, such a very fine machining process is not required in the first-mentioned conventional surface acoustic wave device comprising the diamond layer or the substrate with the diamond layer formed thereon, the Al electrode, and the ZnO piezoelectric thin film layer. Such a surface acoustic wave device is disclosed, for example, in Japanese Unexamined Patent Publication (A) No. 83078/1993.
In the second-mentioned conventional surface acoustic wave device using the single crystal piezoelectric substrate, acoustic impedances of the metal thin film electrode differ between a portion where the electrode fingers are laid and a free surface portion where no electrode finger is laid. This results in occurrence of acoustic reflection of a surface acoustic wave at the electrode fingers.
Likewise, in the first-mentioned conventional surface acoustic wave device comprising the diamond layer or the substrate with the diamond layer formed thereon, the Al electrode, and the ZnO piezoelectric thin film layer, an acoustic impedance of the electrode portion of the Al electrode is not matched with an acoustic impedance of the free surface portion. Such discontinuity of the acoustic impedance results in occurrence of the acoustic reflection.
If addition, let an interdigital electrode structure of single electrode fingers having a width of .lambda./4 be used in the first-mentioned conventional surface acoustic wave device comprising the diamond layer or the substrate with the diamond layer formed thereon, the Al electrode, and the ZnO piezoelectric thin film layer. In this event, the acoustic reflection at the electrode portion causes a ripple to occur in a main lobe of an attenuation characteristic of the device. In order to eliminate such acoustic internal reflection, it has been proposed to use an interdigital electrode structure of electrode fingers having a width of .lambda./8. However, a merit of a high propagation velocity can not be effectively utilized.